Piston-cylinder combination for a combustion engine

ABSTRACT

A combustion engine with compression-initiated combustion comprises at least one cylinder in which a piston is movable axially between a lower dead center point and an upper dead center point, with a dead volume between the cylinder and the piston at the upper dead center point. The dead volume comprises at least two substantially separate spaces in the piston and/or the cylinder at the upper dead center point which are designed to ensure that combustion is initiated at different times.

FIELD OF THE INVENTION

The present invention relates to a combustion engine withcompression-initiated combustion with a piston and cylindercooperatingly shaped for achieving combustion at selected times.

STATE OF THE ART

Conventional diesel engines have an inherent problem of high NO_(x)emissions due to high combustion temperatures which occur in limitedportions of the combustion chamber because combustion takes place alonga combustion front of limited extent.

In combustion engines of the HCCI type, this problem has been greatlyalleviated by using a substantially homogeneous fuel/air mixture tocause substantially simultaneous combustion throughout the combustionchamber, resulting in substantially lower combustion temperatureslocally and hence substantially reduced or even no NO_(x) formation.

However, ideal HCCI combustion throughout the combustion chamber entailsproblems of severe mechanical stress on the various parts of thecombustion engine and significant noise problems.

OBJECTS AND MOST IMPORTANT CHARACTERISTICS OF THE INVENTION

Accordingly, one object of the present invention is to provide acombustion engine which utilises the advantages of the HCCI engine. Aparticular object is to provide a combustion engine of the kindmentioned in the introduction in which the stress and noise problems arereduced.

These objects are achieved in a combustion engine of the kind mentionedin the introduction by providing the piston with dead volume comprisingat least two separate spaces located at the upper center point of thepiston in the cylinder for causing combustion to be initiated atdifferent times.

Controlled lengthening of the period of combustion in the combustionchamber of the cylinder is thereby achieved, resulting in significantadvantages in that the engine will no longer be subject to a stress peakof significant amplitude. The stress problems are thereby reduced andthe noise problems will likewise be alleviated by the whole combustiontaking place simultaneously.

Providing the piston crown with recesses corresponding to the separatespaces makes manufacture easier and reduces costs.

Differentiated cooling of wall portions belonging to the separate spacesis an effective way of controlling the initiation of combustion asdesired so as to achieve desirable delay of combustion in certain partsof the combustion chamber.

Providing thermal insulation for at least some wall portions pertainingto some of the separate spaces achieves higher temperatures incorresponding separate spaces and earlier initiation of ignition inspecifically said space(s), while walls to spaces without thermalinsulation reach lower temperatures and therefore cool the correspondingspace(s), thereby in principle causing later initiation of ignition.

Designing the separate spaces so that the volume/surface area ratio isvaried results in corresponding control of the initiation of ignition,in which respect it may in general be stated that a high volume/surfacearea ratio causes earlier initiation of ignition than a lowvolume/surface area ratio in operating conditions which are otherwisealike.

Connection between the various separate spaces which are arranged fordisparate initiation of ignition as above (insulation, cooling,volume/surface area ratio), via a small gap or narrow channel, may bearranged to provide a delaying effect due to the time which thecombustion takes to spread via the gap or channel from the space inwhich combustion takes place first to the next space.

In addition, differentiated cooling may be applicable, usually in theform of increased cooling in the actual gap or channel. In this case,boundary layer effects also cause natural concentration differences asregards the fuel concentration in thin layers and more extensive regionsrespectively.

It is advantageous to position the separate spaces in such a wayrelative to fuel injectors, fuel mixture intakes, air intakes etc. thatdifferent fuel concentrations occur in the various separate spaces. Thisresults in different times of initiation of ignition.

Further advantages are achieved by further aspects of the inventionwhich are indicated by the following description of an embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail on the basis ofembodiments with reference to the attached drawings, in which:

FIG. 1 depicts schematically a detail of a combustion engine accordingto the invention,

FIG. 2 depicts on a larger scale a section through part of a pistoncrown for a combustion engine according to the invention, and

FIG. 3 depicts in a manner corresponding to FIG. 2 another version of apiston crown for a combustion engine according to the invention.

DESCRIPTION OF EMBODIMENTS

FIG. 1 depicts schematically a section through a cylinder belonging to amulti-cylinder combustion engine of the HCCI type intended, for example,to power a heavy vehicle, e.g. a truck or a bus. In an engine of theHCCI type, fuel ignition is initiated by compression in the same way asin a diesel engine except that the fuel is premixed with combustion air,preferably to a homogeneous mixture, before initiation of ignition inthe same way as in an Otto engine. All of the engine's cylinders are ofsimilar design, so the remainder of this description will cover only oneof its cylinders, although the attached drawings depict variousembodiments of cylinders and pistons.

FIG. 1 depicts a cylinder 2 which has a reciprocating movable piston 3arranged in it in a conventional manner which is capable of movingaxially between a lower dead centre point and an upper dead centre pointand is connected to a crank arrangement incorporating a connecting rod 4and a crankshaft 5 for transmitting driving power from the engine to anoutput shaft.

An inlet duct 6 for supply of air and an exhaust duct 7 for removal ofexhaust gases are connected to the cylinder 2, and these ducts have atleast one inlet valve 8 and one exhaust valve 9 respectively arranged inthem. The cylinder 2 also has a nozzle discharging into it which belongsto a fuel injector 10 which is oriented for injecting fuel directly infothe cylinder 2 in the region of a piston crown hollow 11 in the piston3. The fuel injector 10 is conventionally fitted in a cylinder headwhich delineates the cylinder 2 upwards.

A homogeneous mixture of fuel and air injected for combustion may beachieved in a number of different ways, e.g. by injecting fuel directlyinto the cylinder 2, preferably relatively early in the cylinder'scompression phase, by means of the depicted fuel injector 10.

The piston 3 is cooled in a conventional manner by oil by means of anarrangement incorporating a nozzle 31 which sprays cooling oil towardsan internal space 30 inside the piston, e.g. along a wall which isillustrated by a broken line 32.

FIG. 2 depicts in section a piston crown belonging to a piston 3′. Whenthe piston 3′ is at its upper dead centre point inside the cylinder 2′,an enclosed dead volume is formed between the piston 3′ and the walls ofthe cylinder 2′ (i.e. the cylinder's cylindrical walls and thecylinder's cylinder head). The piston crown is provided with twoseparate spaces, one of these being a first separate space 12 which isprovided with a bowl-shaped lower delineating surface 14. This surface14 is provided with thermal insulation 15 to prevent heat beingconducted away from this region.

The first separate space 12 also comprises a transition region 16 whichis delineated by a planar conical surface on the piston crown. Theresult is a tapering in the axial direction as viewed radially outwardsto an annular edge portion 17 which constitutes a radial inner boundaryof an annular volume which constitutes a second separate space 13 whichis thus situated peripherally relative to (radially outside) the firstseparate space 12.

Within the region of the edge portion 17, which is preferably gentlyrounded, there is a narrow gap between the piston and the cylinder headsituated above it when the piston 3′ is at its upper dead centreposition. The gap is designed in such a way that combustion initiated inthe first separate space 12 spreads with limited velocity to the secondseparate space 13. The result will be delayed initiation of ignition inthe second separate space 13 and hence, in total, a lengthened period ofcombustion of the fuel/air mixture totally enclosed in the cylinder. Inaddition, each of the separate spaces has a respective volume and isdelimited by a respective delineating surface and there is a ratiobetween the volume and the delineating surface of the respectiveseparate spaces that is selected to cause control initiation ofcombustion.

The underside of the piston crown has a recess 28 arranged in it toreceive cooling oil in the same way as described with reference to FIG.1, thereby causing cooling of the surface region peripherally on the topof the piston. This means that the second separate space has a lowertemperature than that in the central part of the piston's first separatespace 12. In addition, the thermal insulation 15 in the bottom of thefirst separate space 12 helps to increase the temperature differencebetween the two separate spaces.

The overall results of this design will be, firstly, delayed initiationof combustion radially outwards in the transition region 16 due to thetaper in the first space 12, and, secondly, further delayed initiationof combustion in the second separate space 13 due to the gap at the edge17 and the cooling by the oil sprayed into the recess 28. The combustioninitiated by the compression will thus take place at different times inthe two spaces.

The general design of the separate second space 13 may be such thatinitiation of combustion only takes place some degrees after thepiston's upper dead centre point, i.e. when the piston has moveddownwards as viewed in FIG. 2.

FIG. 3 depicts a piston 3″ of somewhat different design which isinserted in a cylinder 2″. In this case there are two centrally situatedfirst separate spaces 18 and 19 which are provided with bowl-shapedlower delineating surfaces 22 and 23 respectively. The latter arethermally insulated in a manner corresponding to that shown in FIG. 2.In this case there may possibly be corresponding recesses in a cylinderhead belonging to the cylinder 2″, as illustrated by broken lines anddesignated 20 and 21 respectively to form more spherically shapedvolumes of the combustion space.

In addition, the crown of the piston 3″ exhibits a number of secondseparate spaces, two of which are depicted and are denoted by refs. 24and 25. When the piston is at its upper dead centre position, thesesecond separate spaces may be in communication with the first separatespaces via narrow gaps in the regions 26 and 27 respectively in a mannercorresponding to that described with reference to FIG. 2. Alternatively,communication may also be arranged by grooves being provided in thepiston crown in this region to form channels between the first separatespaces and the second separate spaces. Such channels are illustrated bybroken lines in the regions 26 and 27. Cooling of the second separatespace may be arranged, analogously with what is described above, by oilspray into a space 29 on the underside of the piston. There may at thesame time also be cooling of the regions 26, 27 of the piston which formgaps or incorporate the channels 26, 27 in the piston.

A multiplicity of second separate spaces may be distributed about thetwo first separate spaces 18 and 19 at different distances from theselatter so that varied delay of ignition can be provided for thesevarious second separate spaces.

The invention may be modified within the scopes of the attached claimsand specialists will appreciate that pistons for combustion enginesaccording to the invention may be designed in different ways to ensurethat the initiation of combustion takes place at different times. Thecylinder head may of course also be designed correspondingly. A furtherfactor which affects the combustion is the ratio between the volume andsurface area of the respective spaces, and the initiation of combustionmay be controlled by suitable adjustment of this ratio.

The combustion engine according to the invention is intended forhomogeneous combustion, and the establishment of a homogeneous fuelmixture in the combustion chamber may be achieved as indicated withreference to FIG. 1. However, it may also be achieved, for example, byintroducing a premixed fuel/air mixture via the combustion engine'sinlet ducts by means of an injector (not depicted) arranged in the inletduct 6, for mixing of fuel with air in the inlet duct 6 before theresulting homogeneous fuel/air mixture is introduced into the cylinder2. It is also possible to combine these various versions. It isnevertheless not necessary that the fuel concentration be homogeneousthroughout the dead space, as alternative embodiments may have theseparate spaces so situated as to achieve differences as regards fuelconcentration.

The invention does not exclude the possibility that the engine accordingto the invention might incorporate means for EGR feedback.

The fuels preferred in connection with the invention are normal fuelssuch as diesel fuel or some other suitable liquid or gaseous motor fuel,e.g. gasoline, ethanol, natural gas or methanol.

1. A piston-cylinder combination for a combustion engine , the combination comprising at least one cylinder, a piston movable axially in the at least one cylinder between a lower dead center point and an upper dead center point, the cylinder and the piston being configured for compression-initiated combustion, a dead volume between the cylinder and the piston at the upper dead center point, the dead volume comprising at least two substantially separate spaces between the piston and the cylinder and which are designed to cause combustion in the cylinder to be initiated at different times.
 2. A piston-cylinder combination for a combustion engine according to claim 1, wherein the piston has a crown which has recesses corresponding to and defining the separate spaces.
 3. A piston-cylinder combination for a combustion engine according to claim 2, wherein the cylinder has recesses corresponding to and defining the separate spaces.
 4. A piston-cylinder combination for a combustion engine according to the claim 1, wherein the piston includes wall portions surrounding the separate spaces, and devices to cool the wall portions differentially.
 5. A piston-cylinder combination for a combustion engine according to claim 1, further comprising thermal insulation at least at one of the separate spaces.
 6. A piston-cylinder combination for a combustion engine according to claim 1, wherein the respective separate spaces define a ratio between the volume and a delineating surface of the spaces which is selected to result in controlled initiation of combustion.
 7. A piston-cylinder combination for a combustion engine according to claim 1, wherein the separate spaces are situated to achieve different fuel concentration in different ones of the spaces.
 8. A piston-cylinder combination for a combustion engine according to claim 1, wherein the engine is an HCCI engine.
 9. A piston-cylinder combination for a combustion engine according to claim 1, further comprising a device for supplying a fuel/air mixture to the cylinder.
 10. A piston-cylinder combination for a combustion engine according to claim 1, further comprising a device for direct fuel injection into the cylinder.
 11. A piston-cylinder combination for a combustion engine according to claim 1, wherein the cylinder has recesses corresponding to and defining the separate spaces.
 12. A piston-cylinder combination for a combustion engine with compression-initiated combustion, the combination comprising at least one cylinder, a piston movable axially in the at least one cylinder between a lower dead center point and an upper dead center point, with a dead volume between the cylinder and the piston at the upper dead center point, the dead volume comprising at least two substantially separate spaces between the piston and the cylinder and which are designed to cause combustion in the cylinder to be initiated at different times, and at least one small gap or narrow channel connecting the separate spaces at the position of the piston's upper dead center point.
 13. A piston-cylinder combination for a combustion engine according to claim 12, wherein at least one of the spaces is centrally situated in the cylinder and at least one of the spaces is peripherally situated and a portion which tapers radially outwards in the axial direction leading from the centrally situated space to the peripherally situated space.
 14. A piston-cylinder combination for a combustion engine according to claim 12, further comprising a delineating surface for the gap or the channel and which is cooled.
 15. A piston-cylinder combination for a combustion engine according to claim 12, wherein the second one of the spaces is annular and extends around the first one of the spaces.
 16. A piston-cylinder combination for a combustion engine according to claim 15, further comprising the first one of the separate spaces being more thermally insulated than the second one of the spaces.
 17. A piston-cylinder combination for a combustion engine according to claim 15, wherein the piston has a crown which has recesses corresponding to the separate spaces.
 18. A piston-cylinder combination for a combustion engine with compression-initiated combustion, the combination comprising at least one cylinder, a piston movable axially in the at least one cylinder between a lower dead center point and an upper dead center point, with a dead volume between the cylinder and the piston at the upper dead center point, the dead volume comprising at least two substantially separate spaces between the piston and the cylinder and which are designed to cause combustion in the cylinder to be initiated at different times, wherein the piston has a crown which has recesses corresponding to and defining the separate spaces, and at least one small gap or narrow channel in the crown of the piston and connecting the separate spaces at the position of the piston's upper dead center point.
 19. A piston-cylinder combination for a combustion engine according to claim 18, wherein at least one of the spaces is centrally situated in the cylinder and at least one of the spaces is peripherally situated and a portion which tapers radially outwards in the axial direction leading from the centrally situated space to the peripherally situated space. 